双重for循环。基于牛客692041258号的代码优化。

import java.util.Scanner;


public class Main {
    public static boolean isSymmetry(String str) {
        int n = str.length();
        for (int i = 0; i < n / 2; i++) {
            if (str.charAt(i) != str.charAt(n - i - 1)) {
                return false;
            }
        }
        return true;
    }
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        while (in.hasNextLine()) {
            String str = in.nextLine();
            int n = str.length();
            int max_len = 1;
            for (int i = 0; i < n; i++) {
                for (int j = n - 1; j > i; j--) {
                    if (isSymmetry(str.substring(i, j + 1))) {
                        max_len = Math.max(j - i + 1, max_len);
                    }
                }
            }
            System.out.println(max_len);
        }
    }
}